http://en.wikipedia.org/wiki/Category:Audio_trackers
basically a tracker is an old style sample sequencer (circa mid 80s to present) that handles midi too, on a rudimentary level. the interface is based around text commands that fly by like a piano roll. each command resides on its own line (or tick, usually at a resolution of 32nd notes). basically a big step sequencer with capabilities as a sampler that may seem basic, but all available tricks used in combination are all anyone uses for sampled phrases.
take a look on the 1st page of this thread for pics i just put up of the tracker im using:
AST is an EMU endorsed 'DOS tracker' which utilizes the EMU8000 chip found on Creative SoundBlaster AWE32 and 64 soundcards in a direct manner, rather than through the shitty Creative Windoz driver... therefore allowing direct control of the sample data and hardware via pattern writing. every step can contain a note value, a velocity value, a note cut off, and any one of about 60-80 commands related to any aspect of the system including parameter automation, tempo, instrument selection, etc. you can resample the instruments output (internally, perfectly trimmed measure), save to disk, edit in an external editor (with plugins, etc), reload and away you go. youre only limited by the internal 20bit buss resolution and your imagination! oh, and no native plugins although there are a few destructive DSP functions in the sample editor... basically very fun to use, if you ask me. not as hard as it looks. you can step input it all, or use a controller in some capacity although i havent tried it yet. no sync options unfortunately, but theres midi out, so you can couple it with a midi patchbay and theoretically control a number of pieces of gear from it. my idea of fun.
yeah yeah i hear you say there are newer tracker style options available. theres just something charming about this one. ill start here and move on when i feel completely comfortable...